Planetary, or epicyclic, gear systems are well known for providing different rotational speeds between the input and the output. In epicyclic gear systems, one of the axles of the gears moves in a circular path. Depending on which part of the planetary gear system is driven with an input torque, the output can be a torque of either a higher rpm or a lower rpm. A typical epicyclic gear system includes two or more planet gears mounted to a carrier, a central sun gear that meshes with all of the planet gears, and an annular ring gear that also meshes with all of the planet gears. If the sun gear is driven with an input torque, then the output is obtained from the planet gear carrier, in which event a reduction in speed between the input and the output is achieved. The output speed is reduced, but the torque is increased. Conversely, if the input torque is applied to the planet gear carrier, the output is obtained from the central sun gear. In this case, the input speed is increased, with a corresponding reduction in output torque.
Often, one of the three components of the epicyclic gear system is maintained stationary, or fixed against rotation, and the other components are allowed to rotate. In this configuration, one of the moving components is the input and the other rotating component is the output. Many times, the ring gear is held fixed against rotational movement. The ratio of input rotation to output rotation is a function of the number of teeth in each gear, and the number of teeth of the component which is maintained stationary.
As with all types of gears, the manufacture thereof involves certain tolerances, or errors, in the various dimensions of the gear components. Machine “runout” is the error in the concentricity of a component that cannot be eliminated, but is inherent in milling machines and the like. The higher precision the machine, the less runout is involved, but the cost of the product is correspondingly increased. The diameter of the gear involves a given error that is tolerated, as does the central bore diameter, the teeth spacing, tooth thickness, root diameter, pitch, etc. In some situations, the dimensional errors in the gear components cause an interference, in that the freedom of rotation is reduced as the tolerances can be additive at certain angular positions of rotation, thereby causing the gears to momentarily bind and require additional power to move the gears past the interfering position. When the gears are large with correspondingly large teeth, the interference can be minimal as the increased meshing between the gear teeth does not cause a significant interference, and thus is generally unnoticeable. However, when the gears are constructed with small diameters and correspondingly small teeth, the interference between out-of-round gears can cause substantial interference, thus requiring additional input power. A non-uniform input power is thus required to cause the gears to rotate at a constant speed. A smooth transfer of power between the input and the output is thus more difficult. This situation is exacerbated in epicyclic gear systems where the planet gears not only rotate, but the axles of the planet gears also rotate, thus providing more opportunities for interferences between the three gear components.
Even when machining or otherwise forming gears with a high degree of accuracy, errors can be introduced into the gear system when assembled together. Often gear systems include different housing parts that house the different gear assemblies. When the gear assemblies are assembled in the respective housings, and when the housings are assembled together, such as by using bolts, there are various inaccuracies in the alignment between the housings, and thus between the subassembly of gears that must mesh together and operate as a system. The assembly errors and inaccuracies can be overcome by manufacturing the housings with a high degree of precision, but this significantly increases the overall cost of the gear system.
A need therefore exists for a technique to overcome the adverse affects of cumulative machining errors in the manufacture of planetary gear systems. Another need exists for a technique to allow one of the planetary gear components to float to reduce interference with the planetary gears. Another need exists for allowing the ring gear to float while engaged with the rotating planetary gears mounted to the carrier. Another need exists for assembling the housings of the gear assembly together without directly bolting the housings together, but by using a floating housing arrangement.